1. Field of the Invention
The present invention relates to a lens fixing device for fixing a lens made of a resin on a base, and an optical pickup device incorporated with the lens fixing device.
2. Disclosure of Related Art
In recent years, a lens made of a resin has been used in an optical device such as an optical pickup device. The resin lens has several advantages such as low cost and less weight, as compared with a conventional glass lens. However, the resin lens is likely to be deformed resulting from a temperature change, as compared with the glass lens. Deterioration of an optical characteristic resulting from the deformation has been a problem to be solved.
FIG. 6A is a diagram showing how a resin lens is fixed, when viewed from the optical axis direction of the lens. In this arrangement, a lens 10 is fixedly attached to a base 20, with an upper surface and a side surface of the lens 10 being in proximity to the base 20. In this arrangement, if the lens 10 is expanded with respect to the optical axis by a temperature rise, the lens 10 is displaced in the direction shown by the broken line arrow, while being pressed by surfaces 20a and 20b of the base 20. As a result, the optical axis of the lens 10 may be displaced from a proper position, and a characteristic of an optical system provided with the lens may be deteriorated. In an optical pickup device or a like device, several micrometers displacement of the optical axis of a lens may fail to secure a sufficient characteristic required for reading.
As a measure for solving the above drawback, there is known e.g. an arrangement as shown in FIG. 6B. In this arrangement, three engaging portions 31, 32, and 33 project radially from a lens holder 30. Surfaces S1, S2, and S3 of the engaging portions 31, 32, and 33 to be contacted with a base 40 are formed to radially extend from the optical axis O. The lens holder 30 is resiliently pressed in lower left direction by a plate spring 50 in a state that the surfaces S1, S2, and S3 are contacted with corresponding surfaces of the base 40. Further, the engaging portions 31 and 32 are fixedly attached to an upper surface of the base 40 by an adhesive 60. Cutaways 31a and 32a are respectively formed in the engaging portions 31 and 32 in the optical axis direction. With the cutaways 31a and 32a, the engaging portions 31 and 32 are allowed to flex in left and right directions.
In the above arrangement, when the lens is expanded with respect to the optical axis O by a temperature rise, the surfaces S1, S2, and S3 of the engaging portions 31, 32, and 33 are slidingly moved over the corresponding surfaces of the base 40. At the time of the movement, forces to be applied to the engaging portions 31 and 32 are absorbed by flexure of the portions corresponding to the cutaways 31a and 32a. Thus, optical axis displacement resulting from lens expansion is suppressed.
However, in the arrangement shown in FIG. 6B, since the cutaways 31a and 32a are formed in the engaging portions 31 and 32, the structure of the lens holder 30 becomes complex. Further, it is necessary to mount the plate spring 50 in a state that the surfaces S1, S2, and S3 are contacted with the corresponding surfaces of the base 40 and to fixedly attach the lens holder 30 by the adhesive 60 in mounting the lens holder 30 on the base 40, which necessitates a cumbersome operation. Furthermore, if the force to be applied from the plate spring 50 and the forces to be applied from the cutaways 31a and 32a are imbalanced at the time of lens expansion, optical axis displacement may occur in left and right directions.